An ETF is an investing tool that is similar to stocks, except that the shares of a given ETF represent an index of stocks, other securities or other investments rather than a single company stock. Similar to mutual funds, ETFs provide an investor with various types of diversity within a single fund. However, ETFs provide the added benefit of lower expenses, greater transparency, better tax efficiency, and flexibility. For example, unlike mutual or index funds, whose shares may only be bought at the end of the day based on that day's closing price or net asset value as of 4:00 pm on any given day, ETF shares may be purchased intraday, at any time during the trading day, in the same way stocks are traded. Examples of ETFs are the Standard & Poor's Depository Receipt (SPDR), otherwise known as spider, that trades as a stock on the American Stock Exchange and is an index of, or otherwise represents, the S & P 500; Diamonds (DIA) that trades as a stock on the American Stock Exchange and is an index of, or otherwise represents, the thirty stocks in the Dow Jones Industrial Average; Cubes (QQQQ) that trades as a stock on the NASDAQ and is an index of, or otherwise represents, the NASDAQ 100. However, such ETFs are self-limiting because they are based on the ETF sponsor's previously selected index that, once selected and approved by the SEC, is thereafter limited to that selected index. Current ETFs are not flexible or responsive enough to investors who seek to capitalize on more sudden movements in the market.